Retractable syringes attempt to reduce the risk of needle stick injuries which can result in the transmission of blood born diseases, such as Hepatitis and Human Immuno-deficiency Virus (HIV).
Retractable syringes may be either manually retractable syringes, or auto-retractable syringes. With manually retracting syringes, after an injection has been administered, the user pulls the plunger away from the leading end of the syringe resulting in the plunger and needle tip being pulled inside the barrel. This significantly reduces the risk of needle stick injuries. Similarly, with auto retractable syringes, when the injection has been administered the piston is pushed away from the leading end of the barrel by a spring or other such biasing means and the needle tip is accordingly retracted into the barrel.
In some instances, when the needle tip has been retracted, the trailing end of the plunger is manually snapped off by the medical practitioner at a designated shear point to reduce the likelihood of the syringe subsequently being reused.
With retractable syringes, a problem exists when the plunger is inadvertently completely withdrawn from the trailing end of the barrel again, exposing the needle tip. This exposes the medical practitioner and other persons to the needle tip, and accordingly increases the risk of needle stick injuries.
In addition, in many instances the plunger may subsequently be reloaded into the syringe barrel, enabling the reuse of the syringe. This is most undesirable on account of the inherent risk of blood borne disease transmission when syringes are not adequately sterilised and then reused. Disease transmission resulting from syringe reuse among intravenous drug users is well known. In addition, in developing nations, syringes are often reused for immunisations or other treatments without adequate sterilisation between patients.
In some types of retractable syringes where the plunger can be withdrawn after the plunger has reached the bottom of the stroke. The plunger is intended to stay in a closed position but may undesirably be tampered with by pulling the plunger back and subsequently reassembling the syringe for further use.